Spin-transition compounds and their use for storing information are already known from Patent Application EP 0 251 043. Generally, the compounds known for this use belong to the group of transition metals such as iron and cobalt and preferably Fe.sub.II or Co.sub.II, associated with a ligand.
Said Patent Application gives the following examples:
a) [Fe(2-amino methyl pyridine).sub.3 ]Cl.sub.2, EtOH PA0 b) [Fe(1,10-phenantholine).sub.2 (NCS).sub.2 ] PA0 c) [Fe(1-propyl tetrazole).sub.6 ](BF.sub.4).sub.2
For storing information these materials use the LIESST phenomenon (Light Induced Excited Spin-State Trapping). The LIESST phenomenon is a molecular process in which a spin transition is induced optically.
The use of the known materials in accordance with the LIESST phenomenon is based on the existence of two potential wells, one corresponding to a low spin state and the other corresponding to a high spin state, separated by a potential barrier.
At a temperature which is lower than a critical temperature Tc of the order of 50K, the low spin state is a stable state. This is known from a publication describing the general state of the technique in this field, entitled "Spin-Transition Molecular System; towards Information Storage and Signal Processing" by J. Zarembowitch and O. Kahn in NEW JOURNAL OF CHEMISTRY, Vol. 15, 1991, pp. 181-190. Page 183 of this publication states the Gibbs equation which is defined by: EQU .DELTA.G.dbd.G.sub.HS -G.sub.LS .dbd..DELTA.H-T.DELTA.S
in which .DELTA.G is the energy variation corresponding to the spin transition of a given quantity of material,
.DELTA.H is the enthalpy variation PA1 T is the temperature PA1 .DELTA.S is the entropy variation. PA1 they can only be used at very low temperatures, PA1 their memory effect is very volatile, PA1 they do not have two stable states but a single one, because they present no hysteresis in the range of temperatures considered.
At a low temperature the enthalpy is predominant and the low spin state is the more stable phase, i.e. the phase of lower Gibbs free energy.
The LIESST phenomenon is described on page 185, column 2 of the above-mentioned publication. First, the compound is illuminated in its stable low spin state with radiation having a given wavelength, for example, by means of a laser beam. This illumination induces unstable excited high spin states. These states decay rapidly via allowed transitions, as is shown in FIG. 9 on page 185 of the afore-mentioned publication. The electrons may follow two relaxation paths. In the first path they fall back directly into the first potential wells via allowed transitions, i.e. at the stable low spin ground state. In the other path they fall into the second potential wells corresponding to a high spin state which is not stable, but metastable. When the compound is at a very low temperature, thermal perturbation will have a minor effect. The electrons thus remain trapped in the high metastable spin state during a period of time which is sufficiently long to accumulate enough energy to exceed the potential barrier. Subsequently they fall back into the first potential wells corresponding to the stable low spin state. The period of time during which the system remains in the metastable state may be of the order of a quarter of an hour.
The compounds utilizing the LIESST phenomenon may have a memory effect which is suitable for storing information.
However, in connection with the LIESST phenomenon, the use of these compounds has several drawbacks:
Nevertheless, they are inscribable and optically readable, because the spin transition is accompanied by a structural change of the molecule, related to a varying metal-ligand bond length. This structural change of the spin state becomes manifest as an abrupt color change.
The known materials are thermally erasable, which is effected spontaneously.
The invention has for its object to provide a medium which is inscribable, readable, erasable and reinscribable at ambient temperature, and can be used for storing, processing and/or displaying information.